In many organic oxidation and fermentation processes air is used to provide the source of oxygen. In order to increase the production rate, the air flow into the reactor vessel is increased and, in addition, the resulting air bubbles within the reaction mixture may be decreased in size, such as by the action of impellers or turbines. The increase in air flow into the reactor increases the amount of oxygen available for the oxidation or fermentation reaction, and the smaller size of the air bubbles increases the surface area to volume ratio of the air bubbles thus serving to increase the rate of oxygen mass transfer out from the air bubbles for dissolution in the reaction mixture and subsequent reaction.
However, there is limit to how much additional air may be passed into the reactor, because, beyond a certain flow, the impeller becomes flooded with gases.
To address this problem, oxygen is provided into the reactor to supplement the air. Because commercially available oxygen has an oxygen concentration several times that of air, a much lower volume of supplemental oxygen need be used, as opposed to the volume of additional air that would otherwise be needed, to provide a comparable level of additional oxygen to supplement the basic air. This helps to address the flooding problem, especially when the supplemental oxygen is provided into the reaction mixture at a distance from the impellers where the air is provided.
While air is relatively inexpensive, the use of oxygen imposes a higher cost to the oxidation or fermentation process. One way to moderate this higher cost is to improve the use efficiency of the supplemental oxygen. One way to achieve this is to reduce the tendency of the oxygen bubbles in the reaction mixture to coalesce with the air bubbles to form larger bubbles of oxygen-enriched air. Typically this is done by providing the supplemental oxygen into the reaction mixture at distance from where the air is provided into the reaction mixture.
It is thus seen that for several reasons commercial oxidation or fermentation reaction processes which employ oxygen to supplement air for reaction source oxygen, provide the oxygen into the reactor at a distance from where the air is provided and, consequently, at a distance from the impellers which are used to break up the air stream into smaller bubbles. Typically this supplemental oxygen is provided into the reaction mixture in a downflowing region within the reactor vessel to assure that it is provided far from the rising air bubbles.
While this conventional system effectively keeps the oxygen from coalescing with the air which would negate to a large extent the advantage of using the supplemental oxygen, this procedure has its own drawbacks. With the provision of supplemental oxygen into a reactor vessel at a distance from where the air is provided, the circulation effect within the vessel is reduced because of the braking action of the supplemental oxygen bubbles which try to rise within the downflowing region of the reaction mixture. This reduces the overall efficiency of the process. Moreover, even with a downward pumping impeller, oxygen bubbles can quickly escape to the reaction mixture surface in a turbulently mixed reactor. Thus, injecting the oxygen away from the bottom of the reactor where the air is introduced reduces the residence time available for the oxygen dissolution.
Accordingly, it is an object of this invention to provide an improved method for providing supplemental oxygen to a reaction mixture to which air is also provided for oxidation or fermentation purposes.